14.Dolphin.

This is a kind-of-scary penis. You wouldn't like it if a dolphin tried to have sex with you. And they do try. They're pretty much permanently horny. For e.g. there was that time one of them masturbated with a decapitated fish and racked up millions of YouTube views as a result.

6.Barnacle.

Really solid penis, definitely gets the job done. And actually the biggest in the animal kingdom, relative to body size. They regrow their penises each year, just before their brief mating season. They grow more muscular penises if the water's choppy.

2.Alligator.

This is a really delightful penis. As this blog post states: "Here are the highlights: it’s permanently erect; it shoots out like toothpaste from a tube; and it bounces back because it basically has a rubber band attached to it."

1.Leopard Slug.

This is a briliant penis. Leopard slugs dangle from some goo they've created, extract their penises from their heads, wrap them round each other to make a weird bell shape, then retract them into their bodies with each other's sperm.